I'm looking for some yaoi manga with the seme being very mild and gentle normally but very...

Maybe "Spicy & Sugary"
Also, you can try some Germany x Italy Hetalia's doujinshis, Germany is usually gentle and stoic, but he's wild in bed xD
(Zaria's doujinshi is very good)

http://www.mangago.zone/read-manga/yatamomo/
http://www.mangago.zone/read-manga/puchitto_hajiketa/
http://www.mangago.zone/read-manga/sabita_yoru_demo_koi_wa_sasayaku/
http://www.mangago.zone/read-manga/elektel_delusion/
http://www.mangago.zone/read-manga/hana_no_migoro_ni/
http://www.mangago.zone/read-manga/hana_no_mizo_shiru/
http://www.mangago.zone/read-manga/brother_shuffle/
http://www.mangago.zone/read-manga/katsubou_no_manazashi/
http://www.mangago.zone/read-manga/keep_out_kisaragi_hirotaka/ shen yi story
http://www.mangago.zone/read-manga/mujihi_na_karada/ and series
http://www.mangago.zone/read-manga/calling_ootsuki_miu/
http://www.mangago.zone/read-manga/fuson_na_hana_o_aisureba/